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Ind. Eng. Chem. Res. 2007, 46, 9032-9036
Catalyst Particle Size of Cobalt/Rhenium on Porous Alumina and the Effect on Fischer-Tropsch Catalytic Performance Erling Rytter,*,† Sigrid Eri,† Torild Hulsund Skagseth,† Dag Schanke,† Edvard Bergene,‡ Rune Myrstad,‡ and Asbjørn Lindvåg‡ Statoil Research Centre, N-7005 Trondheim, Norway, and SINTEF Materials and Chemistry, N-7465 Trondheim, Norway
This paper addresses the effect of catalyst particle size distributions on Fischer-Tropsch catalytic performance for cobalt catalysts promoted by rhenium on alumina or modified alumina as catalyst support. The catalyst particle size will affect diffusion of reactants as well as of products. Some key findings are that the C5+ selectivity is rather constant for particle sizes up to ca. 400 µm and that the selectivity reduction thereafter is largely explained by an increase in the methane yield as the effective H2/CO ratio increases. In the whole range of particle sizes investigated, the olefin to paraffin ratio of the light products decreases linearly as the diffusion length becomes longer. Moreover, this ratio is the same for all catalysts in spite of a large variation in selectivity to higher hydrocarbons. It follows that readsorption of olefins and continued chain growth only plays a minor role. The roles of added water and conversion level are also discussed. Introduction Fischer-Tropsch (FT) catalyst systems based on supported cobalt are widely studied and applied for conversion of synthesis gas. They provide high activity, high selectivity to long-chain paraffins, and low-water-gas shift activity. Cobalt in the reduced state is the active component in the H2/CO reaction. The catalyst activity is largely dependent on the degree of reduction of the metal precursor and the shape and size of the metal particles, which control the number of catalytically active sites that are available (dispersion). It has also been shown by Bezemer et al.1 that the TOF (turnover frequency, i.e., the number of CO insertions per active site and time) is constant for Co particles larger than 5-6 nm by using an inert carbon nanofiber as support. The type and structure of the support influence the dispersion and therefore activity for these systems. The most frequently used supports described in the literature are alumina, silica, and titania. These materials behave differently when cobalt is deposited,2 and, moreover, there are huge differences in properties within each class of support. This manifests itself in variations in activity, but to a larger extent in the polymerization probability, e.g., as detected by the C5+ selectivity, and stability, e.g., toward reoxidation by water.3-5 Alumina is probably the most commonly investigated support for Co-FT catalysis. It is well-established that interaction between the different phases containing cobalt and/or aluminum plays an important role in the FT chemistry. For example, it is commonly assumed that a fraction of the cobalt forms, under normal activation conditions, an unreducible cobalt aluminate spinel covering most of the surface.6 However, the actual shape of the cobalt particles or their oxide precursors is unknown, and no good model exists for describing the pore structure of the support, not to mention how these interact with each other. The average properties of alumina, on the other hand, such as pore size distribution, surface area, average pore diameter, and * To whom correspondence should be addressed. Tel.: + 4799165709. Fax: + 47-73584965. E-mail:
[email protected]. † Statoil Research Centre. ‡ SINTEF.
pore volume, can easily be measured. It is found that the pore geometry influences the catalytic properties, in addition to factors such as the purity of the support and the phase.7,8 An increase in cobalt crystallite size and degree of reduction with increasing pore diameter was also found when using a silica support.9 Although γ-alumina is the most common catalyst carrier, a number of transition phases occur before thermodynamically stable R-alumina is reached. It has been shown that hightemperature calcination accompanied by changes in pore geometry and partial transformation of γ-alumina to R-alumina gives a catalyst with a significant increase in selectivity.10,11 Such a treatment significantly reduces the alumina surface area, from typically 150-200 to 5-15 m2/g, and the dispersion and distribution of cobalt thereby possibly becomes restricted by the low available surface area. The catalyst particle size is expected to influence product selectivity for conditions where diffusion of reactants or primary products plays a role. Iglesia and co-workers12 have developed a model for diffusion control. The main findings reported were that by increasing the transport limitations, first an increase in C5+ is seen due to readsorption and continued chain growth of primary formed olefins. Then the selectivity decreases as the pores become depleted in CO relative to the faster diffusing H2. Thereby a typical volcano plot is seen. The present study results in a modification of this model. In a review by van der Laan and Beenackers on kinetics and selectivities in the Fischer-Tropsch process,13 they have shown that a pure diffusion/readsorption model does not explain observed trends in the olefin to paraffin ratio (o/p). It is well-known that the o/p ratio strongly decreases with chain length, even at a flat surface with no diffusion,14 presumably due to an increase in physisorption and solubility with chain length. Recently, Shi and Davis15 studied o/p ratios as a function of chain length by a H2/D2 switching technique and showed that the chain length dependency previously found essentially is a reactor residence time effect. However, the effect of particle size is not mentioned in the latter paper, and this is in focus in the present study. We use propane and propene as probe molecules, thereby minimizing chain length effects in o/p studies.
10.1021/ie071136+ CCC: $37.00 © 2007 American Chemical Society Published on Web 11/22/2007
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Experimental Section Catalyst Preparation. Three catalysts containing 20 wt % Co and 0.5 wt % Re (calculated assuming full reduction after hydrogen activation) on γ-alumina were prepared by one-step incipient wetness from an aqueous solution of cobalt nitrate hexahydrate and perrhenic acid. The supports are Puralox type materials from Sasol GmbH. A fourth catalyst containing 12 wt % Co and 0.5 wt % Re was prepared similarly using a modified support. This support was made by calcination of γ-alumina at 1140 °C for several hours. The impregnated catalysts were dried for 3 h in an oven kept at 110 °C followed by calcination at 300 °C for 16 h. Fischer-Tropsch Tests. The Fischer-Tropsch reactions were conducted in a fixed-bed reactor (stainless steel, 10 mm inner diameter). The apparatus is fully automated with four reactors in parallel. The samples were sieved (53-90 µm), and ca. 1 g was diluted with inert silicon carbide particles (4.0 g, 75-150 µm) in order to improve the temperature distribution along the catalytic zone. An aluminum jacket was placed outside the reactor to secure isothermal conditions. The catalyst was reduced in situ in hydrogen at ambient pressure while the temperature was increased at 1 °C/min to 350 °C. After 16 h of reduction, the reactor was cooled to 170 °C. The system was then pressurized to 20 bar, and synthesis gas of molar ratio H2/ CO ) 2.1 with 3% N2 as an internal standard was added. The temperature was then increased slowly to the reaction temperature of 210 °C. Space velocity was adjusted according to a reactor model to give a predicted carbon monoxide conversion level between 45 and 50% after 100 h time on stream (TOS). Liquid products were removed in a cold trap, while heavy hydrocarbons were collected in a heated trap. The effluent gaseous product was analyzed for hydrogen, nitrogen, carbon monoxide, carbon dioxide, water, and C1 to C9 hydrocarbons using an HP5890 gas chromatograph equipped with a thermal conductivity detector (TCD) and a flame ionization detector (FID). The C5+ selectivity was calculated by subtracting the amount of C1-C4 hydrocarbons and carbon dioxide in the product gas mixture from the total mass balance. Activity is reported as the hydrocarbon formation rate (ghydrocarbon/(gcatalyst h)). To check for particle external mass-transfer limitations, experiments with different catalyst loading, but constant diluted bed heights, were performed. The catalyst space velocity was unaltered, thereby operating at varying linear velocities. Selectivities at constant conversion measured after 100 h TOS show minimal variations. Pore Diameter, Pore Volume, and Surface Area Measurements. Nitrogen adsorption-desorption isotherms were measured with a Micromeritics TriStar 3000 instrument, and the data were collected at liquid nitrogen temperature, 77 K. The samples (0.1-0.3 g, 53-90 µm) were outgassed at 300 °C overnight prior to measurement. The surface area was calculated from the Brunauer-Emmett-Teller (BET) equation, while the total pore volume and the average pore size were calculated from the nitrogen desorption branch applying the BarrettJoyner-Halenda (BJH) method. Results and Discussion In the work by Iglesia and co-workers,12 a volcano type plot of selectivity as a function of catalyst particle size was interpreted as a combination of diffusive readsorption of primary olefins that for the largest particles were superseded by an increase in H2/CO due to limitations in CO diffusivity. A
Figure 1. C5+ selectivity as a function of catalyst particle size for three alumina type supports at ca. 45% conversion. Open symbols represent 10% conversion. Table 1. Pore Characterization material (wt % Co; wt % Re)
pore volume (N2; mL/g)
surface area (m2/g)
pore diameter (nm)
S1: narrow pore γ-alumina S2: wide pore γ-alumina S2*: wide pore γ-alumina S3: R-alumina
Supports 0.51 0.85 0.72 0.09
194 163 170 16
7.3 15.5 12.0 ∼200
cat1: narrow pore γ-alumina cat2: wide pore γ-alumina cat2*: wide pore γ-alumina cat3: R-alumina
Catalysts 0.31 0.60 0.49 0.08
150 127 135 21
7.0 13.1 12.0 ∼150
consequence was a model whereby olefin readsorption, followed by continuous chain growth, depended on the site density of cobalt. The denser the cobalt sites, the more probable is readsorption. Catalyst and Catalyst Supports. In this work three different types of supports out of a total of seven investigated ones are compared. They are further specified in Table 1, and the main characteristics are narrow pore γ-alumina, wide pore γ-alumina (two slightly different samples), and R-alumina. We clearly can see that the three types of alumina span a large space of properties, and these again result in rather different cobalt distribution and crytal sizes (see Table 2) as well as catalytic behavior. Hydrocarbon Selectivity at Constant Conversion. As it is known that the conversion level influences the selectivity, presumably due to increasing partial pressure of water,10,17 experiments were run at a constant conversion of CO of ca. 45% at 100 h TOS. The open symbols in the figures are for a cat2 type catalyst, but with only 10% CO conversion, clearly showing the effect of conversion. The influence of water on the selectivity will be discussed in more detail separately. By examining Figure 1, an essentially constant C5+ selectivity is found up to 300-350 µm for any of the investigated catalysts. Thereafter a decrease occurs, very marked for the 650 µm samples. This reduction in C5+ selectivity can be ascribed to a diffusion effect in that hydrogen diffuses more rapidly than carbon monoxide, thereby giving an effective higher H2/CO ratio in the inner part of the larger particles. It is in itself remarkable that all the catalysts follow the same trend with approximately the same break point, although the absolute selectivity level may vary. The pore structure is significantly different from one sample to another, but this evidently has no bearing on the location of the break point. It is natural to conclude that the diffusion is pore size independent and therefore is a bulk effect
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Figure 2. Methane selectivity as a function of catalyst particle size for three alumina type supports at ca. 45% conversion. Open symbols represent 10% conversion.
Figure 3. Cn selectivity (% C) as a function of catalyst particle size for a wide pore γ-alumina support (cat2) at ca. 45% conversion. Open symbols represent 10% conversion.
Table 2. Cobalt Characterizationa
catalyst cat1: narrow pore γ-alumina cat2: wide pore γ-alumina cat2*: wide pore γ-alumina cat3: R-alumina a
Co surface (m2/g) 11.7
Co size Co3O4 size (chemisorption; (XRD; nm) nm)
agglomerate size (nm) 77 (55-100)
10.2 10.7 4.8
151 (65-300) ∼200b
Cat1 and cat2 data from Borg et
al.16 b
16.0
11.1
21.5
12.7
19.4
12.2
24.5
16.3
Densely packed agglomerates.
characteristic of the transport length only. The catalysts also are different in terms of cobalt loading, metal (oxide) particle size, and support acidity etc., but it is not expected that these parameters will influence the relative H2 and CO diffusion. The results are in variance with the reports of Iglesia and coworkers,12 who found a volcano type of plot as a function of a structural Thiele modulus. A slight increase up to ca. 220 µm, however, can be inferred. Note, however, that although we have covered a wide spread in pore sizes, supports with a significant contribution of microporosity may behave differently. This typically is the case for zeolite type supports. In Figure 2 it is shown that the C5+ selectivity loss for the largest particles is coupled to an increase in methane formation. This complies with the accepted mechanism that hydrogenation of methyl species is taking place by a mechanism independent of chain growth and that termination to methane is very sensitive to the hydrogen partial pressure. On the other hand, termination of growing chains to C2, C3, C4, and C5 evidently is not nearly as sensitive to the hydrogen partial pressure as the level of total Cn is rather constant with particle size, but with a downward trend (Figure 3). If methane had been excluded from the C-selectivities, no apparent trend can be seen. Clearly, no breakpoint around 300 µm occurs for the C2-C5 components. A detailed product analysis over a wide carbon number range indeed shows that the Andersen-Schultz-Flory (ASF) R-value is rather constant with particle size. For a catalyst on γ-alumina with medium pore size, i.e., between cat1 and cat2, the R-value decreased as 0.902 (46 µm), 0.909 (225 µm), 0.885 (363 µm), and 0.889 (638 µm). In a control experiment with small catalyst particles of cat2, the H2/CO ratio was varied between 1.7 and 2.5 (1.7; 1.9; 2.1; 2.3; 2.5). Indeed, the methane carbon selectivity increased strongly, linearly from 6.39 to 9.08 at 42% conversion, whereas the sum SCH4 + S5+ only exhibited a moderate reduction from
Figure 4. C3 olefin to paraffin ratio as a function of catalyst particle size for three alumina type supports at ca. 45% conversion. Open symbols represent 10% conversion.
93.75 to 92.99%, but still indicating a slight effect on the propagation probability. There certainly is an effect on the olefin to paraffin ratio, to be discussed in the next section, but the observed reduction in this ratio from 2.97 to 2.12 is much more moderate than the experienced effect of particle size. As described, there clearly are overall trends in hydrocarbon selectivity that apply to all three catalyst types. The most distinct difference, however, between the catalysts is the C5+ level for small catalyst particles where the trend is R-alumina > largepore γ-alumina > small-pore γ-alumina. This observation correlates with the pore size,7 but also with the pore volume, cobalt oxide crystal size, and cobalt aggregate size.15 It is obvious that the C5+ differences are present for small particles with no apparent pore diffusion, and further investigations are needed to sort out the basic cause and effect. Another possible difference between the catalysts seen in Figure 1 is that the particle size effect is more moderate for the wide pore γ-alumina. This is confirmed for two other large pore γ-aluminas as well. Olefin Selectivity. The olefin to paraffin ratio is illustrated for C3 in Figure 4. The same linear trend with catalyst particle size is found for C4 and C5 as well. Again it is important to compare data at the same conversion level as it is known that olefin hydrogenation is inhibited by water; refer to the open symbols in the figure. It is remarkable that the olefin/paraffin ratio steadily is reduced through the whole series of particle sizes, also in the range where the C5+ selectivity is constant. Evidently, the o/p ratio is not linked to the C5+ selectivity under the conditions investigated here. Another way of expressing this is that the o/p ratio is independent of the chain growth probability for different catalysts. It is generally assumed that readsorption of olefins is an important part of the FischerTropsch reaction network,12 and certainly, addition of olefins
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to the syngas feed has some effect on the long-chain paraffin yield. In a first approximation, however, olefins apparently are hydrogenated by a mechanism separate from the chain growth. The hydrogenation does not depend on the pore structure or on the reactivity of the support but is clearly proportional to the residence time in the pores. If then the variation in the properties of the support is not important, this indicates that the hydrogenation reaction takes place at the cobalt particles. Indeed, separate studies on propene hydrogenation show that the rate is proportional to the available surface area of reduced cobalt.18 Note that the discussion above has been independent of one striking observation: the absolute o/p values are the same for all catalysts in spite of the variations in chain growth probability. This is consistent with a reaction scheme whereby the ratio between the chain termination rates to olefins and paraffins, respectively, is constant for the investigated catalysts and independent of the propagation probability. This is in line with the near independency of o/p with carbon number found recently.15 Thus, readsorption of olefins hardly can play a major role for the FT reaction, whereas independent diffusion controlled hydrogenation apparently takes place. A prerequisite for this model is that the total olefin plus paraffin Cn selectivities are fairly constant with particle size for each individual catalyst. This interpretation is supported by the total C3 and C4 depicted in Figure 3, as discussed above. Another interpretation of the data that at first sight seems plausible should be mentioned. If olefins are readsorbed into further chain growth accompanied by a higher degree of hydrogen-assisted termination for the large catalyst particles, we could observe a constant ASF R as well as reduction in the o/p. The main evidence against such a model is that the hydrogen effect only plays a role for the largest particles (refer to the methane selectivity), whereas incorporation should take place for smaller particles as well. A volcano plot should have been seen and also a break point in the o/p curves. We therefore conclude that hydrogenation of olefins is an independent reaction closely linked to the residence time in the pores and that this reaction probably takes place at reducible cobalt sites. Effect of Conversion Level. Numerous reports have shown a dependence on the selectivity to higher hydrocarbons with the conversion level, as confirmed by the 10% CO conversion data in Figures 1-3 for the wide pore γ-alumina type support. The most noticeable difference in process conditions with conversion is that the partial pressure of water increases. Indeed, the C5+ response has been ascribed to surface coverage of water/ hydroxyl that suppresses termination reactions.19 When the conversion is decreased, we see that the altered C5+ selectivity is due to an increase both in methane and in light hydrocarbons, in other words in a reduced polymerization probability, or R-value. The described trends with particle size also are seen at the low conversion levels. This also applies to the o/p ratio (see Figure 4), although the absolute ratio is enhanced at the low conversion. For a system of consecutive reactions this is what is expected. At a given temperature and partial pressures, the initial o/p ratio is fundamental to cobalt Fischer-Tropsch catalysis, irrespective of the support. For low conversions, the consecutive olefin hydrogenation will not be particularly pronounced. Extrapolation to zero conversion under nondiffusion-limited conditions, i.e., for small particles, then gives the fundamental Co FT o/p ratio. It is significant that olefins are the dominating primary products, and chain termination therefore should be nearly independent of the H2/CO ratio, i.e., of partcicle size.
Figure 5. (a) C5+ selectivity as a function of CO conversion for different catalyst particle sizes (µm) of a wide pore γ-alumina support (cat2) and (b) corresponding C3 olefin to paraffin ratios.
Figure 6. C3 olefin to paraffin ratios as a function of CO conversion for two catalyst particle sizes (72 and 363 µm) of a wide pore γ-alumina support (cat2*) with and without water added to the feed.
In Figure 5, results for the wide pore γ-alumina are shown as a function of the conversion level. Again it is demonstrated that the C5+ selectivity is constant with particle size except for the largest particles. Further, the trend with conversion is independent of particle size, indicating that there are minimal diffusion effects of the generated water. In Figure 5b, the o/p ratio diminishes gradually with the particle size of the catalyst as seen previously. The dependency with conversion follows the expectation of consecutive reactions, i.e., olefin generation followed by olefin hydrogenation. No apparent effect of the generated water can be detected, as this should suppress olefin hydrogenation and give the reverse trend. Note that the present investigation is performed at a constant total pressure. It therefore can be argued that due to the higher partial pressure of hydrogen at low conversions, C5+ should decrease and the o/p ratio should be reduced. The latter is contrary to the experimental findings and shows that total pressure effects are inferior to the influence of conversion. In fact, separate experiments have shown that the selectivities are insensitive to the total pressure in the actual range of process conditions. Influence of Water. Adding steam to the feed should, according to the above effect of conversion level, result in a higher polymerization probability, and this is indeed observed
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for all conditions investigated. Besides, it is known that steam suppresses the olefin hydrogenation reaction.18 Therefore, at a constant conversion level steam is expected to increase C5+, reduce the CH4 and light hydrocarbons levels, and increase C3 o/p. In Figure 6 the latter is clearly illustrated for two particle sizes of the alternative wide pore γ-alumina catalyst. As the conversion increases, the thereby higher water vapor pressure is expected to enhance the o/p ratio, but this is contrary to the observations, clearly due to the competing effect of the consecutive olefin hydrogenation reaction. Water is a stoichiometric product of the reaction and as such will accumulate in the pores for large catalyst particles if diffusion restrictions are imposed. Such accumulation should increase C5+, parallel to what is seen for water addition, but no such increase with particle size has been detected. It therefore is confirmed that water diffusion at most gives a secondary effect. Probably water freely can equilibrate inside the catalyst particles. Overall Model and Conclusions The following can be inferred from Fischer-Tropsch experiments conducted on cobalt/alumina catalysts in an integral fixedbed reactor at conditions typical for high yield of long-chained paraffins (H2/CO ) 2.1; Ptotal ) 20 bar; T ) 210 °C): (a) Olefins are hydrogenated on cobalt independently of the FT reaction; i.e., the olefin to paraffin ratio is constant for variations in cobalt loading, site density, and cobalt dispersion, etc. The hydrogenation is enhanced by bulk pore diffusion of light olefins even for particles in the 70-120 µm range. The hydrogenation is independent of pore characteristics. (b) Readsorption of olefins with continued FT chain growth does not play a detectable role under the conditions investigated. (c) The primary olefin/paraffin ratio of a given carbon number, i.e., before any secondary hydrogenation, is characteristic of the FT reaction on cobalt and is independent of catalyst type. (d) For catalysts with particle size above ca. 300 µm, methane yield is increased, and not, to a first approximation, the FT propagation probability. (e) Although water is shown in the literature to inhibit olefin hydrogenation, consecutive olefin hydrogenation at increasing conversion levels gives a more pronounced effect than generated water on the o/p ratio. (f)- The effects of water are essentially independent of particle size; i.e., there is no apparent diffusion limitation for water in the particle range investigated. To maximize coproduction of (light) olefins to a FischerTropsch wax plant with cobalt catalyst means an operation with low conversion and small catalyst particles or egg-shell type catalyst as well as steam added to the feed. Literature Cited (1) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P.
Cobalt Particle Size Effects in the Fischer-Tropsch Reaction Studied with Carbon Nanofiber Supported Catalysts. J. Am. Chem. Soc. 2006, 128, 3956. (2) Storsæter, S.; Borg, Ø.; Blekkan, E. A.; Holmen, A. Study of the Effect of Water on Fischer-Tropsch Synthesis over Supported Cobalt Catalysts. J. Catal. 2005, 231, 405. (3) Borg, Ø.; Storsæter, S.; Eri, S.; Wigum, H.; Rytter, E.; Holmen, A. The Effect of Water on the Activity and Selectivity for γ-Alumina Supported Cobalt Fischer-Tropsch Catalysts with Different Pore Sizes. Catal. Lett. 2006, 107, 95. (4) Jacobs, G.; Das, T. K.; Patterson, P. M.; Li, J.; Sanches, L.; Davies, B. H. Fischer-Tropsch Synthesis. XAFS Studies of the Effect of Water on a Pt-Promoted Co/Al2O3 Catalyst. Appl. Catal., A 2003, 247, 335. (5) Schanke, D.; Hilmen, A. M.; Bergene, E.; Kinnari, K.; Rytter, E.; Ådnanes, E.; Holmen, A. Study of the Deactivation Mechanism of Al2O3Supported Cobalt Fischer-Tropsch Catalyst. Catal. Lett. 1995, 34, 269. (6) Lee, W. H.; Bartholomew, C. H. Multiple Reaction States in CO Hydrogenation on Alumina-Supported Cobalt Catalysts. J. Catal. 1989, 120, 256. (7) 7) Rytter, E.; Eri, S.; Schanke, D. Fischer-Tropsch Catalysts. Patent Application: International Publication No. WO04/043596 A2, 2004. (8) Eri, S.; Rytter, E. Cobalt and Rhenium Containing Fischer-Tropsch Catalysts. Patent Application: International Publication No. WO06/010936 A1, 2006. (9) Saib, A. M.; Claeys, M.; van Steen, E. Silica Supported Cobalt Fischer-Tropsch Catalysts: Effect of Pore Diameter of Support. Cat. Today 2002, 71, 395. (10) Eri, S.; Kinnari, K. J.; Schanke, D.; Hilmen, A.-M. Fischer-Tropsch Catalyst with Low Surface Area Alumina, Its Preparation and Use Thereof. Patent Application: International Puiblication No. WO 02/47816 A1, 2002. (11) Schanke, D.; Eri, S.; Rytter, E.; Aaserud, C.; Hilmen, A.-M.; Lindvåg, O. A.; Bergene, E.; Holmen, A. Fischer-Tropsch Synthesis on Cobalt Catalysts Supported on Different Aluminas. Stud. Surf. Sci. Catal. 2004, 147, 301. (12) Iglesia, E.; Reyes, S. C.; Soled, S. L. Reaction-Transport Selectivity Models and the Design of Fischer-Tropsch Catalysts. In Computer-Aided Design of Catalysts; Becker, E. R., Pereira, C. J., Eds.; Dekker: New York, 1993; pp 199-257. (13) van der Laan, G. P.; Beenackers, A. A. C. M. Kinetics and Selectivity of the Fischer-Tropsch Synthesis: A Literature Review. Catal. ReV.-Sci. Eng. 1999, 41, 255. (14) Kuipers, E. W.; Vinkenburg, I. H.; Oosterbeek, H. Chain Length Dependence of R-Olefin Readsorption in Fischer-Tropsch Synthesis. J. Catal. 1995, 152, 137. (15) Shi, B.; Davis, B. H. Fischer-Tropsch Synthesis: The Paraffin to Olefin Ratio as a Function of Carbon Number. Catal. Today 2005, 106, 129. (16) Borg, Ø.; Eri, S.; Storsæter, S.; Blekkan, E. A.; Wigum, H.; Rytter, E.; Holmen, A. Fischer-Tropsch Synthesis over γ-AluminaSupported Cobalt Catalysts: Effect of Support Variables. J. Catal. 2007, 248, 89. (17) Hilmen, A.-M.; Lindvåg, O. A.; Bergene, E.; Schanke, D.; Eri, S.; Holmen, A. Selectivity and Activity Changes upon Water Addition During Fischer-Tropsch Synthesis. Stud. Surf. Sci. Catal. 2001, 136, 295. (18) Aaserud, C.; Hilmen, A.-M.; Bergene, E.; Eri, S.; Schanke, D.; Holmen, A. Hydrogenation of Propene on Cobalt Fischer-Tropsch Catalysts. Catal. Lett. 2004, 94, 171. (19) Frøseth, V.; Storsæter, S.; Borg, Ø.; Blekkan, E. A.; Rønning, M.; Holmen, A. Steady-State Isotopic Transient Kinetic Analysis (SSITKA) of Co Hydrogenation on Different Co Catalysts. Appl. Catal., A 2005, 289, 10.
ReceiVed for reView August 20, 2007 ReVised manuscript receiVed September 21, 2007 Accepted September 21, 2007 IE071136+
